An interview with Jim Christy: How “defective” images revealed Pluto as a double planet

Charon’s discoverer says when he first spotted a blob on Pluto’s side in 1978, he thought there’d been an explosion on the planet.
By | Published: June 9, 2015 | Last updated on May 18, 2023
James Christy points to the “defective” photographic plate that he used to find Pluto’s moon Charon in 1978.
On June 22, 1978, US Naval Observatory (USNO) astronomer James Christy was getting ready to take some well-deserved time off. He and his wife, Charlene, were moving from an apartment to their newly purchased home in the Maryland suburbs outside Washington, D.C. So Christy caught up on all the tasks he could find around the office and checked in with his supervisor, Robert Harrington, to see if there was anything else before he left for the week.

The supervisor reached into his desk and pulled out a set of Pluto images that Christy had requested and waited so long to get he’d forgotten he was expecting them.

Christy’s job at USNO was to measure the moons of Uranus and Neptune using images collected by the observatory’s 61-inch Strand Telescope in Flagstaff, Arizona. Those measurements were forwarded on to NASA’s Voyager program so that Jet Propulsion Laboratory engineers could plot them. But Christy realized that he could easily extend his efforts to Pluto, which USNO hadn’t emphasized much in several years. For some reason, even though his proposed observations were approved, the folks in Flagstaff hadn’t gotten around to taking them.

Harrington handed over six photographic plates that were taken in April and May of that year.

“The plates had been marked by whoever looked at it in Flagstaff as ‘image defective,’ which is probably why Bob didn’t measure them himself or didn’t look at them,” says Christy. “I started to measure the plates. The way the images looked, you wouldn’t expect to get a decent measurement of Pluto.”

It was obvious the planet was blurry. It was elongated.

An explosion on Pluto
“I did the usual effort of going around and measuring the star images, and they were perfect,” Christy says. “There was no problem with the stars, so that meant whatever was wrong with Pluto, it had to be real.”

The astronomer amplified the plates under a microscope and quickly realized that in one of the images, Pluto was actually elongated on the opposite side from the other frames.

“So, at first, I thought there was an explosion on Pluto, but you wouldn’t expect that to last for a month. And when it was on the opposite side of some of the images, then I thought about the satellite idea.”

Late in the morning that same day, Christy took his find to a colleague he often bounced ideas off of, and the pair looked through a star atlas for anything that might have moved through the frame to cause the “defect.” They quickly picked out a star that Pluto passed near and Christy moved on with his day, giving up on the idea of a satellite.

Part of the reason why Christy and other astronomers hadn’t looked for satellites of Pluto earlier is that one of the pillars of planetary science, Gerard Kuiper, who discovered Uranus’ moon Miranda and Neptune’s moon Nereid, said he’d already looked. Kuiper was also one of Christy’s professors at the University of Arizona, and he spent two nights on the 200-inch Hale Telescope on Mount Palomar in the 1950s observing Pluto.

“It’s strange,” Christy says. “You accept it. This is your hero, and he says it’s not there. So I had never looked. Nobody ever did.”

A quick confirmation
After lunch on the day of the discovery, Christy settled into a routine measuring job for about 10 minutes before it dawned on him that the odds of a background star interfering at the exact time of observation were minuscule. It had to be a moon.

Next, the astronomer started digging up old photographic plates of Pluto. He found the same elongation in images from 1970 and 1965. Harrington told Christy to measure the position angles of his moon while he calculated an orbit based on the light variations seen in both the old and new images.

“We didn’t contact each other until we were both done, and we compared the opposition angles of his predictions and my measurements, and they were all right on six plates in a three-day period, which was amazing,” Christy says. 

Even more amazing, Christy says he had been convinced the astronomers in Flagstaff had overexposed their images of Pluto in the past when he requested his observations. He asked them to halve their usual 4-minute exposure period. And while the astronomers gathered Christy’s images much later than expected, they happened to observe Pluto at the ideal time of year and perfect position in the sky.

The USNO contacted a colleague in Chile and received confirmation that there was indeed an elongation on Pluto at the point Christy and Harrington predicted. So, the observatory announced its find to the world July 8, 1978.

Pluto and Oz?
As discoverer, Christy earned the naming rights to Pluto’s companion. He and Charlene were driving to her parents’ wedding anniversary when they started chatting about what to call it. His first choice was Oz. Then, he thought he could name it for his wife. Physicists were constantly finding new particles in those days and naming them by simply adding “‘on” to the end of their chosen word. So, he concocted “Charon.”

When he went to the observatory, the other astronomers had already come up with Persephone, Pluto the god’s wife. Christy says he thought their suggestion was so good it was automatic, and he went off to move into his new home with help from the guys at the observatory.

“We got moved into the house, and there was no power in the house, so we had no refrigeration or lights, and when we finished up, it was about 10 o’clock at night and I was totally exhausted,” Christy says. “I went to sleep immediately, and then a strange thing happened — I think the strangest thing in my life. About right after midnight, I woke up I was thinking ‘In the morning, I have to tell Charlene that I can’t name the moon after her.’”

He says that he got up, half asleep, and stumbled through the house with a flashlight looking for the boxes of books.

“I found the dictionary and opened it up, and it said Charon, and in Greek mythology the boatman that sails across the river Hades into the domain of Pluto, and I said ‘OK,’ and I just staggered back to bed.”

He reluctantly told the crew of his decision and the rest is history. To honor Charlene, astronomers now pronounce the moon like “Sharon” instead of the traditional Greek pronunciation, which sounds more like “Karen.”

Christy says he still likes Oz.

“That would go along with Hollywood Pluto, and it would have been great for kids,” he says. “I think that would have been a better choice. I think of all the kids who learn about Pluto and Oz, and there would be a lot of imagination, since nobody would ever see it.”

Harrington, who is often credited as Charon’s co-discoverer for his role in the find, had married James and Charlene’s friend, Betty-Jean Maycock, who actually introduced the Christys. The Harringtons named their daughter Ann Charon.

Bob Harrington died tragically of cancer at the age of 51 in 1993. Christy left the USNO in 1982 so he could return to Arizona. He ultimately retired early in Flagstaff, where he still lives. 

What does he expect to see when New Horizons reveals his moon for the first time next month?

“I have no ideas,” he says. “I hope it’s a big surprise and not a flat gray ball.”

Eric Betz is an associate editor of Astronomy. He’s on Twitter: @ericbetz.